You mention article 18 of the UN declaration on human rights:
"Article 18
Everyone has the right to freedom of thought, conscience and religion; this right includes freedom to change his religion or belief, and freedom, either alone or in community with others and in public or private, to manifest his religion or belief in teaching, practice, worship and observance."
But if you invoke Article 18, you also have to mention Article 29 paragraph 2:
"In the exercise of his rights and freedoms, everyone shall be subject only to such limitations as are determined by law solely for the purpose of securing due recognition and respect for the rights and freedoms of others and of meeting the just requirements of morality, public order and the general welfare in a democratic society."
So freedoms can be limited by law, if the freedom of one group is detrimental to the freedoms of others.
So your knife thing is covered. It's a case of weighing up rights of different groups and deciding on the greater good. Freedom of religion is not infinite. It can be, and really often has to be, limited. And those limits will change as society changes.
And this is where I think at can get difficult. Because if you try to limit the rights of one group, they can use the same arguments to limit the rights of others. It is a proverbial can of worms.
For Example. This is in the Christian Bible :
1Ti 2:12: But I suffer not a woman to teach, nor to usurp authority over the man, but to be in silence. (KJV)
I would say that is against the law of the land in the UK
So, if one group goes for the religious freedom of another group care is needed. Because they, or anyone really, can legitimately go after the religious freedoms of others.